DEF Tank indication problem

[Beachnsand]

DEF tank indicates Full on dashboard and about half on the local indication. DEF tank is about 1/3 to 1/2 currently (dipped the tank to check level). seems that the DEF tank level sending unit is not working correctly.

Any suggestions on how to troubleshoot this issue?

Does the ignition need to be in "ACC" position for the local DEF tank indicator to indicate? (Local indicator looks to be electrical and not a mechanical type of indicator)

[Smosher]

Ignition has to be in the on position to make the indicator work on the gage on the tank